The first time Ben Affleck worked up the courage to go in a store and buy a box of condoms, it was a rather traumatizing experience for the future Hollywood hunk!

“I was so scared,” recalled Ben, who was 18 years old then. “I wondered: ‘Are they going to call my mom?’ I went into the store, walked up to the shelf and grabbed the box. I didn’t stand there looking too long. There were two registers. A man was at one, a woman at another. Of course, the woman’s register came up first. I felt so self-conscious and idiotic, but it was all over in a second. I put them down and she looked at me with this stoic face and rang them up and put them in a bag, and I ran out of that store really fast!”

The Cambridge homey’s squirmy confession is contained in a 1990 teen sex guide called “Risky Times: How to be AIDS Smart and Stay Healthy,” written by former WBZ medical reporter Jeanne Blake.

Blake, as you may know, was married to Matt Damon’s dad, Kent, at the time, and she enlisted the help of six of her stepson’s BFFLs for her “Guide For Teenagers.”(Matt, 20 at the time, aged out — a circumstance he is, no doubt, rather thankful for!)

“As you know, kids learn best from their peers, and I wanted to include the voices of young people who could be model messengers to other kids,” Blake told us the other day.

The group included Affleck and Aaron Stockard, another of Matt and Ben’s childhood buds, whom you may recognize as the co-screenwriter of Ben’s two flicks, “Gone Baby Gone” and “The Town.”

“I knew that this was a special group of kids,” Blake recalled, “They were such a blast to be around. I’m not surprised that the whole group has gone on to be successful.”

Jeanne admitted she hadn’t thought much about her book lately until it surfaced during Damon’s recent appearance on “The Tonight Show with Jay Leno” a few weeks back.

Leno whipped out the book, showed a picture of the slightly geeky teenage Affleck, and proceeded to grill Matt about it.

“Yes, that’s Ben. And all the sex he wasn’t getting,” Damon teased as Leno showed off a black-and-white photo of the then-gawky teen.

“There’s some good dirt on Ben in there, I’m sure,” Matt continued, which naturally sent us on a quest to find the book, which we did, although it is currently out of print.

Some other pearls of wisdom from Ben contained in the tome include:

• “If you are a guy, you take care of it, you know what I’m saying? If you are a girl, you should insist on it no matter what. Don’t be embarrassed.”

• “I don’t look down on people who don’t have sex, sometimes I think they are smarter . . .”

• “What’s hard about being a teen is, I guess, caring so much about what people think of you.”

• “There are so many things I want to do in my life. I want to be given the chance to become someone. I want to see how the story ends.” Pretty well, as it turns out!

“He was such a great kid,” Blake recalled. “So funny and so smart — like he still is.”

The former TV reporter said she wrote the book during the height of the AIDS crisis because she had been lecturing in schools and found that most kids didn’t have even the most basic facts about protecting themselves.

“I thought if I could put together all the facts and all the myths and tell them through stories, then I could be in more places than I could possibly physically be,” she said.

Blake subsequently left TV and now works for Words Can Work writing and producing films, DVDs and Web series aimed at helping parents talk to kids about a host of health-related issues like obesity, and even bullying.

“The AIDS book was the beginning of what became a very satisfying career,” she said.

Blake said she still has no idea how her manual ended up on “The Tonight Show,” but she knows that Damon didn’t bring it with him.

“Matt seemed surprised,” she said. “I really wonder how they put two and two together and ended up with it sitting there, but it was fun.”